1. Field of the Invention
This invention relates to a zoom lens suitable for the photographing optical system of a photographing apparatus using a solid state image pickup device such as a CCD sensor or a CMOS sensor.
2. Description of Related Art
With the higher function of a video camera, an electronic still camera or the like using a solid state image pickup device, there is desired the compatibility of high performance and compactness.
In the camera of this kind, it is necessary to dispose various optical members such as an optical low-pass filter and a color correcting filter between the rearmost portion of a lens system and the image pickup device and therefore, a lens system having a relatively long back focal distance is required. Further, in the case of a camera using an image pickup device, in order to avoid shading, there is desired a lens having a good telecentric characteristic on an image side, that is, having an exit pupil sufficiently separate from an image plane.
As a compact zoom construction, there have heretofore between proposed various zoom lenses of a so-called short zoom type having two lens units of negative-positive refractive power. In these optical systems of the short zoom type, a second lens unit of positive refractive power is moved to thereby effect a magnification variation, and a first lens unit of negative refractive power is moved to thereby effect the correction of an image point position resulting from the magnification variation. Further, mention may be made of examples in which for higher performance and downsizing, as described in Japanese Patent Publication No. 07-3507 (corresponding U.S. Pat. No. 4,810,072), Japanese Patent Publication No. 06-40170 (corresponding U.S. Pat. No. 4,647,106), etc., a third lens unit of negative or positive refractive power is disposed on an image side to thereby effect aberration correction. These lens systems, however, are designed chiefly for 35 mm film photographs and therefore can hardly be said to be compatible in a back focal distance and a good telecentric characteristic required of an optical system using a solid state image pickup device.
As a zoom lens system satisfying a back focal distance and a telecentric characteristic, mention may be made of an optical system comprising three lens units of negative-positive-positive refractive power described in Japanese Patent Application Laid-Open No. 63-135913 (corresponding U.S. Pat. No. 4,838,666), Japanese Patent Application Laid-Open No. 07-261083, etc., but this optical system has suffered from the defects that the number of the constituent lenses of each lens unit is relatively great, that the full length of the lens is great and that the manufacturing cost is high.
Also, in the example described in Japanese Patent Application Laid-Open No. 07-261083, a first lens unit of negative refractive power is moved to thereby effect focusing on a short-distance object and therefore, coupled with the movement for zooming, there has been the defect that mechanical structure is complicated.
Also, U.S. Pat. No. 4,999,007 discloses a zoom lens system comprising three lens units of negative-positive-positive refractive power in which each of a first lens unit and a second lens unit is comprised of a single lens. This zoom lens system, however, has suffered from the defect that the entire lens system becomes large because the full length of the lens at the wide angle end is relatively great and further the first lens unit and a stop at the wide angle and are greatly separate from each other and therefore the incidence height of an off-axial ray is great and the diameter of a lens constituting the first lens unit is increased. Also, the number of the constituent lenses of each of the first lens unit and the second lens unit is one and therefore, aberration correction in each of these lens units has been insufficient. Particularly, the fluctuation of transverse chromatic aberration during zooming is liable to occur in the first lens unit wherein the fluctuation of the height of the off-axial ray from an optical axis is great, but since the first lens unit is comprised of a negative lens, the correction in the lens unit is not done, and this leads to the problem that in the entire system as well, the fluctuation of transverse chromatic aberration is great.
Also, U.S. Pat. No. 4,824,223 discloses an optical system for a projector comprising three lens units of negative-positive-positive refractive power. In this lens, a first lens unit comprises a negative lens and therefore, aberration correction in the lens unit is not done and the variable power ratio is other order of 1.7.
Also, the assignee of this application discloses in Japanese Patent Application Laid-Open No. 2000-111798 (corresponding U.S. Pat. No. 6,308,011) a photo-taking lens comprising three lens units of negative-positive-positive refractive power. In this photo-taking lens, the securement of a lens back necessary for inserting a filter or the like and a telecentric characteristic necessary for a solid state image pickup device are made compatible and moreover, a relatively compact zoom lens is achieved.
In the zoom lens disclosed in Japanese Patent application Laid-Open No. 2000-111798, however, the three lens units are moved on different loci during a zooming and therefore, movable lens units are many and mechanical structure becomes liable to be complicated, and there have been problems in respect of the bulkiness of a lens barrel and cost.
As an example of a three-unit construction of negative-positive-positive refractive power in which a third lens unit is made stationary, there is the aforementioned Japanese Patent Application Laid-Open No. 07-261083, but in this example, a second lens unit is xe2x88x921 in its magnification and has a zooming position in which focusing becomes impossible and therefore, it is necessary to effect focusing by a first lens unit or a third lens unit. In a case where focusing is effected by the third lens unit, there must be adopted a construction in which all of the three lens units are movable, and this leads to problems in respect of the bulkiness of a lens barrel and cost. In a case where focusing is effected by the first lens unit, the focusing becomes fore lens focusing and therefore, when the shortest image pickup distance is shortened, an increase in the diameter of the fore lens results, and therefore, there is a problem in respect of downsizing.
As examples of the three-unit construction of negative-positive-positive refractive power in which the first lens unit is made stationary, mention may be made of Japanese Patent No. 3035830 (corresponding U.S. Pat. No. 5,270,863), Japanese Patent Application Laid-Open No. 8-304704 (corresponding U.S. Pat. No. 5,668,668), Japanese Patent No. 2552861, Japanese Patent Application Laid-Open No. 10-213745, Japanese Patent Application Laid-Open No. 2000-137164 and U.S. Pat. No. 5,015,077.
U.S. Pat. No. 5,015,077 is a lens system for microfilm projection and the number of lenses is as great as the order of 13, and this leads to a problem in respect of downsizing.
In Japanese Patent Application Laid-Open No. 08-304704, magnification variation is effected by a composite system of a second lens unit and a third lens unit, and this example is of a construction in which during the zooming from the wide angle end to the telephoto end, a third lens is greatly moved toward the object side with a second lens unit, and therefore suffers from the problem that the shaft length of the third lens unit is great.
In Japanese Patent Application Laid-Open No. 2000-137164 and Japanese Patent No. 2552861, a second lens unit is comprised of one or two lenses and does not include a negative lens, and there is a problem in respect of aberration fluctuation resulting from zooming.
In Japanese Patent Application Laid-Open No. 10-213745, a second lens unit is comprised of a positive lens and a negative lens, but the positive refractive power of the second lens unit is formed by this one positive lens and therefore, as compared with a triplet or like construction, this example is inferior in aberration correction, and has a problem as a lens system particularly for a camera using a high-pixel image pickup device.
In Japanese Patent No. 3035830, a second lens unit is comprised, in succession from the object side of three positive, positive and negative lenses, and the last surface of the second lens unit is a concave surface facing the object side. A combination of this concave surface and the positive refractive power of a third lens unit has the action of keeping an exit pupil away from an image plane, but if an attempt is made to reduce the interval between the second lens unit and the third lens unit to thereby shorten the full length while keeping the exit pupil away from the image plane, it is necessary to make the refractive power of the concave surface of the second lens unit and the third lens unit great, and the correction of off-axial aberration becomes difficult and therefore, it is difficult to make the shortening of the full length and high performance compatible while keeping the exit pupil away from the image plane.
So, the present invention has as its object the provision of a zoom lens system in which a minimum number of lens units are moved to thereby effect zooming and provided a simple mechanical construction and various aberrations are corrected well over an entire zoom area and which has a short full length and is compact and yet keeps an exit pupil sufficiently away from an image plane.
In order to achieve the above object, a zoom lens system according to an aspect of the present invention is provided, in succession from the front (if in a photographing optical system, the object side) to the rear (if in the photographing optical system, the image side), with a first lens unit of negative optical power, a second lens unit of positive optical power and a third lens unit of positive optical power. In case of zooming from a short focal length end (the so-called wide angle end) to a long focal length and (the so-called telephoto end), the second lens unit is fowardly moved and the third lens unit is moved along the locus which is convex toward the image side or a part of it. In case of the zooming, the first lens unit does not move, and the third lens unit describes such a movement locus that it is located rearmostly at other zoom position than the short focal length end.
The first lens unit has a negative lens of which the rear surface is a concave surface and a positive lens of which the front surface is a convex surface. The second lens unit has a negative lens of which the rear surface is a concave surface and a positive lens disposed rearwardly of the negative lens.
Further, this zoom lens system satisfies the following condition:
0.7 less than (e12t+e23t)/(e12w+e23w) less than 1.4, 
where e12w is the interval between the first lens unit and the second lens unit at the short focal length end, e23w is the interval between the second lens unit and the third lens unit at the short focal length end, e12t is the interval between the first lens unit and the second lens unit at the long focal length end, and e23t is the interval between the second lens unit and the third lens unit at the long focal length end.